DYNAMICAL GEOLOGY 79 
DYNAMICAL GEOLOGY 
Origin of Desert Ranges of Mexico. Desert geology acquaints 
us with many new ways of regarding geological phenomena. 
Their consideration from the new angle throws a quite different 
light upon some of the relief features which were long ago thought 
to be fully and easily explained. A welcome addition to our 
knowledge of the geology of Mexico is the brief communication 
presented at the recent Wilkes-Barre meeting of the American 
Institute of Mining and Metallurgical Engineers by Charles 
Laurence Baker, on the “Catorce District in the State of San 
Luis Potosi.’’ 
Catorce is an old and important silver camp of Mexico, con¬ 
cerning which we have had too little information. Mr. Baker 
does not fall into the usual scholastic blunder of describing the 
mountain range at Catorce as a ridge due to block-faulting, al¬ 
though the Sierra de Catorce is one of the desert ranges which, 
at least on the American side of the border, has been sweepingly 
condemned to this origin, regardless of their individual geological 
structure. 
Mr. Baker observantly explains: ‘‘As is usual in desert regions, 
the heavy-bedded limestones are the most resistant to erosive 
forces. Folded into anticlines they form the mountain ranges; 
and the intermountane valleys and basins, originally carved out 
of less resistant shales, clays, marls and sandstones, are covered 
by debris, forming a mantle with surface gradually rising towards 
the hills.” A lucid, true and appreciative description of the desert 
ranges of northern Mexico, as contrasted with the usual twaddle 
about block-ridges, and the chatter of horst and graben. 
Spurr. 
Minimum Span of Isostatic Effect. When the conception of 
isostatic compensation, that most brilliant geological hypothesis 
of our century, saw birth amidst the isolated desert ranges of the 
